The present exemplary embodiments relate to materials for promoting vertical alignment of liquid crystals. It finds particular application in conjunction with the vertical alignment of liquid crystals on SiOx and other substrates, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Liquid crystal (LC) materials are rod-like molecules which have different optical properties along their long and short axes. The molecules exhibit some long range order so that locally they tend to adopt similar orientations to neighboring molecules. The local orientation of the long axes of the molecules is referred to as the “director”. There are three types of LC materials: nematic, cholesteric (chiral nematic), and smectic. For a liquid crystal to be used in a display device, it must typically be made to align in a defined manner in the “off” state and in a different defined manner in the “on” state, so that the display has different optical properties in each state. Two principal alignments are homeotropic or vertical (where the director is substantially perpendicular to the plane of the cell walls) and homogeneous or planar (where the director is inclined substantially parallel to the plane of the cell walls). In practice, planar alignments may be tilted with respect to the plane of a cell wall, and this tilt can be useful in aiding switching.
Homeotropic alignment of liquid crystals on substrates is useful in liquid crystal displays, rear projection TVs, optical communication, and other applications. Usually in this mode liquid crystals with negative dielectric anisotropies are employed because they are electrically switchable when molecules are aligned vertically. Numerous methods have been developed to achieve vertical alignment of liquid crystals.
For example, sputtered SiO2, evaporated SiOx, SiOx evaporation followed by alcohol treatment, and polyimide thin layers have all been used. Among these methods, SiOx evaporation and SiO2 sputtering attracts much interest because they are non-contact and rub-free.
LC homeotropic alignment is a difficult process to control, typically using a chemical treatment of the surface, such as lecithin or a chrome complex. These chemical treatments may not be stable over time, and may not adhere very uniformly to the surface to be treated. Homeotropic alignment has been achieved by the use of special polyimide resins such as those developed by Japan Synthetic Rubber Co. These polyimides need high temperature curing which may not be desirable for low glass transition plastic substrates. Inorganic oxide layers may induce homeotropic alignment if deposited at suitable angles. This requires vacuum processes which are subject to the problems discussed above in relation to planar alignment. Another possibility for producing homeotropic alignment is to use a low surface energy material such as PTFE. However, PTFE gives only weak control of alignment angle and may be difficult to process.
However, it is still difficult to vertically align many liquid crystals on them. We found a group of materials that promote vertical alignment if mixed with other liquid crystals at an appropriate concentration range.
Previous researchers have reported that liquid crystals with negative dielectric anisotropy tend to align homeotropically on deposited inorganic surfaces while those with positive dielectric anisotropy tend to align homogeneously. Minhua Lu, K. H. Yang, T. Nakasogi, and S. J. Chey, SID Digest, 2000, pp 446-449.
Specifically, the researchers found that several liquid crystals with negative dielectric anisotropy could be aligned homeotropically on evaporated SiOx and sputtered SiO2 substrates. The paper further pointed out that due to the smoothness of the SiOx surface, steric forces play only minor roles in the alignment of liquid crystals. Therefore, the van der Waals forces are the keys. This research also suggested a model to explain this effect by taking the induced dipole-dipole interaction into account in calculating the anisotropy of liquid crystal polarizabilities. This model was expressed using the equation:
  V  ∝                              ɛ          lc                -        1                              ɛ          lc                +        2              *                            ɛ          SiO                -        1                              ɛ          SiO                +        1              *          r              -        6            
This model predicts that liquid crystals with positive dielectric anisotropy should align homogeneously on SiOx surfaces while liquid crystals with negative dielectric anisotropy should align homeotropically on SiOx surfaces. This has been experimentally proved in many cases.
However, it has been shown that while liquid crystals with moderate values of a negative dielectric anisotropy typically align as predicted, there are at least some liquid crystals that have a large negative dielectric anisotropy that are difficult to align vertically on SiOx or even on polyimide substrates. This is problematic because large negative values of the dielectric anisotropy are desired to enhance the electro-optical response of devices.
It is desirable to have a more controllable and manufacturable alignment method for LC devices. Thus, it would be helpful to develop a method for promoting vertical alignment of any liquid crystal exhibiting negative dielectric anisotropy on inorganic substrates. and to be able to control the type of alignment through the simple addition of a specific alignment promoting material to the liquid crystal.